One question I haven't seen answered: I haven't done any of the Thronebreaker achievements on my PS4. Will I still be able to do them after the shutdown Gwent and how will my PS4 Gwent collection interact with the TB one if that makes any sense? And will I have to buy TB again on GOG or will they provide your account with a "free" TB as compensation when we already bought it for PS4?
Final question: Do I have to transfer my account before they shutdown the game on PS4. If you don't do that, will you lose your account or something?
Once your account is transferred, you can no longer claim the Thronebreaker rewards from the console. I do hope CDPR offers console users some compensation, like a free copy of Thronebreaker on GOG for those that already own the game on the PS4.
As for what happens to your account after the game is shut down, I cannot say for sure. I think it's probably still saved somewhere, but you can no longer access it.